This invention relates to the demodulation of auxiliary low frequency channels in digital transmission systems, and is particularly but not exclusively suited to optical transmission systems.
Several methods of providing auxiliary low frequency channels in high speed digital systems are known. For example, it is known in pulse code modulation systems to add one extra bit to each code group, the extra bits forming what may be termed a "distributed" code for an extra channel. Another arrangement is to impose a form of pulse position modulation of a selected bit position in a p.c.m. code. In optical systems it is known to impose low level low frequency amplitude modulation on digitally modulated laser.
The present invention is concerned with a system in which low frequency digitally coded signals for an auxiliary channel are superimposed on a high frequency main digital signal. The auxiliary channel may for instance by a binary channel at baseband, say FSK (frequency shift keyed) or PSK (phase shift keyed) or even ASK (amplitude shift keyed) at, typically, 70 kb/s on a 4 MHz channel whilst the main channel(s) is scrambled binary at 565 mb/sec.